The present application relates to the heat distribution and vacuum tube arts. It finds particular application in conjunction with the rechannelling of by-product heat in rotating anode x-ray tubes and will be described in conjunction therewith.
A high power x-ray tube typically includes a thermionic filament cathode and an anode which are encased in an evacuated envelope. A heating current, commonly on the order of 2-5 amps, is applied through the filament to create a surrounding electron cloud. A high potential, on the order of 100-200 kilovolts, is applied between the filament cathode and the anode to accelerate the electrons from the cloud towards an anode target area. The electron beam impinges on a small area of the anode, or target area, with sufficient energy to generate x-rays. The acceleration of electrons causes a tube or anode current on the order of 5-600 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat which heats the anode white hot.
In high energy tubes, the anode rotates at high speeds during x-ray generation to spread the heat energy over a large area and inhibit the target area from overheating. The cathode and the envelope remain stationary. Due to the rotation of the anode, the electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam.
The anode is typically rotated by an induction motor. The induction motor includes driving coils, which are placed outside the vacuum envelope, and a rotor with an armature and a bearing shaft, within the envelope, which is connected to the anode. When the motor is energized, the driving coils induce electric currents and magnetic fields in the armature which cause the armature and other portions of the rotor to rotate.
The temperature of the anode can be as high as 1,400xc2x0 C. Part of the heat is transferred through the anode to the rotor, which includes the armature and the bearing shaft, which are both connected to the anode through a stem portion of the rotor. Heat travels through the bearing shaft to the bearing races and is transferred to the lubricated bearing balls carried by the races. The lubricants on the bearing balls become hot and tend to evaporate. X-ray tube bearing life is critical to high performance tube operation. Conduction and radiation from the hot target heat the bearing shaft and this heating can evaporate or otherwise degrade the ball bearing lubricant, leading to a rapid degradation of the bearing surfaces and premature tube failure.
Because x-ray tubes operate in a vacuum requiring low vapor pressure materials, petroleum-based lubricating compounds cannot be used. Thus, it is common in the industry to use solid metal lubricants, such as lead, on the bearing races. The evaporation of lead lubricant from a bearing race accelerates rapidly over 350xc2x0 C. These temperatures can be reached in the bearing, primarily during processing, and also during field life. The evaporation of lubricant leads to a rapid degradation of the bearing surfaces and premature tube failure. In an x-ray tube, the front bearing race is physically closer to the hot target than the rear bearing. Because of this, the front bearing runs about 100xc2x0 C. hotter than the rear bearing and fails at a much higher rate than the rear bearing.
To reduce lubricant evaporation, silver lubrication on the ball bearings is sometimes used in place of lead. Silver has a lower vapor pressure than lead and can be run at least 100xc2x0 C. hotter than lead. However, silver lubrication has a number of drawbacks. It tends to react with the bearing steel if it becomes too hot and causes grain boundary cracking and premature failure of the bearing. Additionally, silver requires more starting and running torque than lead, due to its lower lubricity. The torque imparts more residual heat into the bearing, through frictional and eddy current induction heating of the bearing and surrounding rotor body components. Silver lubricating material also creates more noise during operation than lead.
In recent years, computerized tomography (CT) gantry rotation speeds have increased from about 60 rpm to 120 rpm. Higher gantry rotation speeds call for x-ray tubes with higher x-ray output densities. Again, increasing the x-ray output increases the heat output. The x-ray tubes typically need larger diameter rotating anodes to handle the increased heat output. This change has resulted in increased stresses on the rotor stem and bearing shaft. One way to reduce these stresses to a non-critical level is to reduce the length of the rotor stem while increasing the cross sectional area. This, however, shortens and widens the heat conduction path from the target to the bearing shaft, resulting in higher thermal transfer.
One way to reduce bearing temperatures is to provide a thermal block to isolate the bearing lubricant from the heat of the target. Many low power tubes do not have a thermal block of any kind. In these tubes, the rotor stem is screwed directly to the bearing shaft. The absence of a thermal block in the system causes the bearing shaft to run at a high temperature, especially during x-ray generation, and this can lead to evaporation of the bearing lubricant and failure of the x-ray tube.
A variety of thermal blocks have been developed for reducing the flow of heat from the anode to the bearing shaft. In one low power design, the rotor stem is brazed to a steel rotor body liner that is then screwed to the bearing shaft. This provides a slightly more thermally resistive path.
Another thermal block that has been used in the industry is known as a top-hat design. A top hat-shaped piece of low thermal conductivity material, such as Hastelloy(trademark) or Inconel(trademark), is screwed onto the hub of the x-ray bearing shaft. The rotor body is then attached to the brim of the top hat with screws, welds, or other fastening means. The thermal path from the rotor body to the bearing is then extended by the length of the top hat. Analysis shows that a 20-50xc2x0 C. temperature decrease may be achieved at the front bearing race when the top hat design is employed.
The top hat design, however, has several disadvantages.
In CT x-ray tubes, the rotating anode assembly is a precision device, with the focal track runout kept to a few tens of micrometers or lower. For high power scanners, the tolerance requirements are even more stringent. Because of the narrow tolerances all of the x-ray tube components are highly precision machined and precisely assembled. The introduction of additional parts in the assembly increases the problems of maintaining a precise alignment. It is clearly much easier to align two components within desired tolerances than to align three or more components. Thus, the additional component introduced in the top hat design makes it more difficult to assemble the rotor to within design tolerances.
The top hat design also reduces the rigidity of the assembly because the additional length of the top hat tends to adds flexibility to the rotor structure. This can lead to excessive deflection of the target and focal spot and associated imaging problems.
Another thermal block has been formed by inserting a washer of a low conductivity material between the bearing hub and the rotor stem. The washer provides a slightly longer thermal path and additional thermal resistance between the rotor stem and the bearing. Thermal analysis shows a 10-20xc2x0 C. reduction in temperature at the front bearing race when a washer is used. The washer thermal block has the same problems as the top hat design with respect to the additional tolerance stack-up. In addition, it is difficult to make the washer thick enough to provide the necessary heat blocking without introducing excessive deflection into the assembly.
Another method of reducing heat flow to the bearing shaft involves reducing the thermal contact areas and increasing the thermal contact resistance between the bearing shaft hub and the rotor stem. This is typically achieved by minimizing the surface area of the bearing hub and using a rough surface, so the contact resistance between the hub and rotor stem is maximized. Most manufacturers have already reduced the bearing contact areas as much as is practical. In a high performance x-ray tube, however, this is often insufficient for maintaining the bearing race temperatures at an acceptable level. The contact areas can only be reduced up to a certain point, beyond which plastic deformation occurs in the parts due to excessive stress. If plastic deformation were allowed to occur, a loose assembly would result, which could fail prematurely from excessive vibration.
Another method of reducing heat flow is to use a spiral groove bearing. The bearing has a series of spiral cuts formed in an exterior surface of the bearing adjacent a bearing race. The spiral groove bearing is a relatively complex, large bearing that employs a gallium alloy to transfer heat. The bearing shaft is limited to a rotational speed of about 60 Hz. This limits operating power of the x-ray tube. Conventional x-ray tube anodes run from 60 Hz to 180 Hz, which allow higher power protocols to be performed.
The present invention provides a new and improved x-ray tube and bearing assembly which overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, an x-ray tube is provided. The tube includes an envelope which defines an evacuated chamber. A cathode is disposed within the chamber for providing a source of electrons. An anode is disposed within the chamber which is struck by the electrons and generates x-rays. A rotor rotates the anode relative to the cathode. The rotor includes a bearing shaft, connected with the anode, including an inner portion and a peripheral portion connected with the inner portion. The peripheral portion defines forward and rear bearing races in an exterior surface. The forward bearing race is closer to the anode than the rear bearing race. The inner and peripheral portions define an annular gap at least partially therebetween, which extends longitudinally into the bearing shaft and spaces a forward end of the inner portion from a forward end of the peripheral portion. The gap extends the path along which heat entering the bearing shaft through the inner portion forward end travels to reach the forward bearing race. Lubricated bearings are received by the forward and rear bearing races.
In accordance with another aspect of the present invention, a bearing member for an x-ray tube is provided. The bearing member includes a bearing shaft, including an inner cylindrical portion and a peripheral portion, connected with the inner portion. The peripheral portion defines at least a first annular bearing race, which is shaped to receive lubricated bearing members therein. An annular gap extends into the bearing shaft generally parallel with the peripheral surface and provides a thermal barrier between a portion of the peripheral portion adjacent the first bearing race and an adjacent portion of the inner portion. A connecting member extends from the inner portion for connecting the bearing member with an anode.
In accordance with another aspect of the present invention, a method of reducing evaporation of a bearing lubricant in an x-ray tube having an anode and a rotatable bearing shaft connected therewith is provided. The shaft defines a bearing race in an exterior surface thereof. The method includes channelling heat entering the bearing shaft from the anode around an annular insulation zone defined in the bearing shaft.
One advantage of the present invention is the provision of an effective thermal block for a bearing shaft of an x-ray tube rotor.
Another advantage of the present invention is that the temperature of the forward bearing race is reduced.
Yet another advantage of the present invention is that the length of the x-ray tube rotor and the number of components to be maintained in precision alignment are not increased.
Yet another advantage of the present invention resides in increased x-ray tube life.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment.